Exhaust heat recovery system

ABSTRACT

An exhaust heat recovery system ( 10 ) includes an exhaust heat recovery heat exchanger ( 18 ) in which heat is transferred from exhaust gas discharged from an internal combustion engine ( 12 ) to an engine coolant for cooling the internal combustion engine ( 12 ), and a heat pump system ( 22 ) for recovering exhaust heat of exhaust gas. The heat pump system ( 22 ) includes a refrigerant circulation passage ( 24 ) for circulating CO2 refrigerant, a heat-absorbing heat exchanger ( 32 ) in which heat is transferred from exhaust gas to the CO2 refrigerant, and a heat-releasing heat exchanger ( 28 ) in which the exhaust heat recovered by heat transfer to the CO2 refrigerant in the heat-releasing heat exchanger ( 32 ) is recovered.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an exhaust heat recovery system applied to avehicle, such as an automobile, to supply heat to heat the vehicle cabinand warm up the engine.

2. Description of the Related Art

A vehicle heat pump system having the following features is available(e.g. heat pump system disclosed in Japanese Patent ApplicationPublication No. H3-90430 (JP-A-3-90430)). The heat pump system includesan exhaust gas heat exchanger and a heat pump circuit. In the exhaustgas heat exchanger, heat of exhaust gas discharged from an internalcombustion engine is recovered, and transferred to a coolant of theengine. The heat pump circuit uses, as a heat source, the outside airand the engine coolant heated in the exhaust gas heat exchanger. If theheating mode is selected when the temperature of the engine coolant islow, the supply of the engine coolant to a heater core is stopped, andthe vehicle cabin is heated using the heat pump circuit.

However, in the aforementioned technology, since the heat pump uses, asthe heat source, the outside air and engine coolant heated in theexhaust gas heat exchanger, further improvement of the heat pumpefficiency for effective heating is desired.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an exhaust heat recoverysystem in which the exhaust heat can be effectively recovered.

An exhaust heat recovery system according to an aspect of the inventionincludes an exhaust heat recovery heat exchanger in which heat istransferred from exhaust gas discharged from an internal combustionengine to a coolant for cooling the internal combustion engine. Theexhaust heat recovery system also includes a heat pump. The heat pumpincludes a circulation passage for a refrigerant, a heat-absorbing heatexchanger in which the heat is transferred from the exhaust gas to therefrigerant, and a heat-releasing heat exchanger in which the heat isrecovered from the refrigerant.

In the exhaust heat recovery system according to the aforementionedaspect, exhaust heat (energy) of exhaust gas discharged from theinternal combustion engine is recovered and transferred to the coolantin the exhaust heat recovery heat exchanger for quickly warming up theinternal combustion engine, heating the vehicle cabin, and the otherpurposes. Further, the exhaust heat of exhaust gas is recovered andtransferred to the refrigerant in the heat-absorbing heat exchanger andused as the heat source to operate the heat pump. The internalcombustion engine is quickly warmed up and the vehicle cabin is heatedby, for example, heat transfer from the refrigerant in theheat-releasing heat exchanger.

Since the heat pump uses exhaust gas as the heat source of the heatpump, that is, since the heat pump uses high-temperature fluid as theheat source, the heat pump efficiency is high. Therefore, exhaust heatis not discharged to the outside of the vehicle when exhaust gas isdischarged, and can be efficiently recovered. The heat pump according tothe invention thus has higher performance than that of the conventionalconfiguration in which the outside air or fluid, such as coolant, atrelatively low temperature is employed as the heat source. Thus, thethermal efficiency of the entire vehicle is improved.

Thus, in the exhaust heat recovery system according to theaforementioned aspect, exhaust heat can be effectively recovered.Further, since exhaust gas is used as the heat source of the heat pump,frost formation is prevented.

In the exhaust heat recovery system according to the aforementionedaspect, the heat-absorbing heat exchanger of the heat pump may bedisposed downstream of the exhaust heat recovery heat exchanger in aflowing direction of the exhaust gas.

In the exhaust heat recovery system according to the aforementionedaspect, exhaust heat of exhaust gas is first recovered and transferredto the coolant in the exhaust heat recovery heat exchanger, and then theexhaust heat that is not recovered in the exhaust heat recovery heatexchanger is recovered in the heat pump. Further, since the remainingheat is transferred from exhaust gas at relatively low temperature tothe refrigerant in the heat-absorbing heat exchanger after the heat ofthe exhaust gas is transferred to the coolant in the exhaust heatrecovery heat exchanger, overheating (i.e. excessive pressure increase)of the refrigerant can be prevented. Thus, the heat pump is protectedagainst high temperature by employing the simple configuration, and theexhaust heat can be effectively recovered using the more compact systemcompared to the system in which, for example, the heat transfer area inthe exhaust heat recovery heat exchanger is expanded.

In the exhaust heat recovery system according to the aforementionedaspect, the coolant may be circulated by a pump driven by a power fromthe internal combustion engine.

The exhaust heat recovery system according to the aforementioned aspectmay further include an exhaust heat recovery switching device that isswitched between an exhaust heat recovery state where exhaust heat ofthe exhaust gas is recovered and a discharge state where the exhaust gasis discharged to an outside. In the exhaust heat recovery system, theexhaust gas may flow through the exhaust heat recovery heat exchangerand the heat-absorbing heat exchanger by switching the exhaust heatrecovery switching device from the discharge state to the exhaust heatrecovery state.

The exhaust heat recovery system according to the aforementioned aspectmay further include a control device that switches the exhaust heatrecovery switching device to the exhaust heat recovery state when atemperature of the coolant is lower than a predetermined temperature,and switches the exhaust heat recovery switching device to the dischargestate when the temperature of the coolant is equal to or higher than thepredetermined temperature.

In the exhaust heat recovery system according to the aforementionedaspect, the heat pump may include a compressor that compresses therefrigerant to which the heat has been transferred in the heat-absorbingheat exchanger. The exhaust heat recovery system may further include acontrol device that switches the exhaust heat recovery switching deviceto the exhaust heat recovery state and operates the compressor when thetemperature of the coolant is lower than the predetermined temperature.

In the exhaust heat recovery system according to the aforementionedaspect, when the temperature of the coolant is equal to or higher thanthe predetermined temperature, the control device may control theexhaust heat recovery switching device to the discharge state andcontrol the compressor to a stopped state.

In the exhaust heat recovery system according to the aforementionedaspect, the heat-releasing heat exchanger of the heat pump may beconfigured so that the heat is transferred from the refrigerant to thecoolant therein.

In the exhaust heat recovery system according to the aforementionedaspect, heat is released into the coolant in the heat-releasing heatexchanger of the heat pump, and therefore all the heat recovered fromexhaust gas is transferred to the coolant. In this way, using thecoolant, the internal combustion engine can be quickly warmed up and airheating (heating of the air for air conditioning) can be achieved.

In the exhaust heat recovery system according to the aforementionedaspect, the heat-releasing heat exchanger of the heat pump may bedisposed upstream of the exhaust heat recovery heat exchanger in thecirculation passage for the coolant.

In the exhaust heat recovery system according to the aforementionedaspect, the exhaust heat is released into the coolant in theheat-releasing heat exchanger of the heat pump system. Since the heat isreleased into the coolant at relatively low temperature before thecoolant is heated in the exhaust heat recovery heat exchanger, the heatpump efficiency is high, thus contributing to improvement of the entireheat recovery efficiency.

The exhaust heat recovery system according to the aforementioned aspectmay further include a heater core that is disposed upstream of theexhaust heat recovery heat exchanger in the circulation passage for thecoolant and heats air for air conditioning. In addition, theheat-releasing heat exchanger of the heat pump may be disposeddownstream of the heater core in the circulation passage for thecoolant.

In the exhaust heat recovery system according to the aforementionedaspect, since the heat is released into the coolant at the lowesttemperature in the circulation passage, that is, the heat is releasedinto the coolant after the heat is released from the coolant to the airin the heater core but before the coolant is heated in the exhaust heatrecovery heat exchanger, the heat pump efficiency is further improved,thus contributing to further improvement of the entire heat recoveryefficiency.

In the exhaust heat recovery system according to the aforementionedaspect, a heat-releasing heat exchanger of the heat pump may beconfigured so that the heat is transferred from the refrigerant to theair for air conditioning therein.

In the exhaust heat recovery system according to the aforementionedaspect, the air for air conditioning is heated in the heat-releasingheat exchanger of the heat pump so that the recovered heat is used forthe vehicle cabin heating. Since the air is directly heated in theheat-releasing heat exchanger, heating start-up time can be shorter thanin the case where the engine coolant is used for heating.

The exhaust heat recovery system according to the aforementioned aspectmay further include a heater core that is provided in the circulationpassage for the coolant and heats the air for air conditioning, and aheater core switching device that is switched between an air supplystate where the air for air conditioning is supplied to the heater coreand a heater core bypass state where the air for air conditioning is notsupplied to the heater core.

In the exhaust heat recovery system according to the aforementionedaspect, when the heater core switching device stops air supply to theheater core, heat is not released from the heater core so as to heat theair. Therefore, the temperature of the coolant staying in the heatercore does not decrease, and warm-up time for the internal combustionengine can be shortened.

The exhaust heat recovery system according to the aforementioned aspectmay further include a heat pump switching device that is switchedbetween an air supply state where the air for air conditioning issupplied to the heat-releasing heat exchanger and a heat pump bypassstate where the air for air conditioning is not supplied to theheat-releasing heat exchanger.

In the exhaust heat recovery system according to the aforementionedaspect, the heater core switching device is switched to the heater corebypass state when the heat pump is operated, and the heat pump switchingdevice is switched to the heat pump bypass state when the heat pump isstopped.

The exhaust heat recovery system according to the aforementioned aspectmay further include: an exhaust heat recovery switching device that isswitched between an exhaust heat recovery state where the exhaust heatof the exhaust gas is recovered, and a discharge state where the exhaustgas is discharged to an outside; and a control device that switches theexhaust heat recovery switching device to the exhaust heat recoverystate and switches the heater core switching device to the heater corebypass state when a temperature of the coolant is lower than apredetermined temperature.

The exhaust heat recovery system according to the aforementioned aspect,which includes the heat pump having a compressor that compresses therefrigerant to which the heat has been transferred in the heat-absorbingheat exchanger. The exhaust heat recovery system may further include anexhaust heat recovery switching device that is switched between anexhaust heat recovery state where exhaust heat of the exhaust gas isrecovered and a discharge state where the exhaust gas is discharged toan outside, and a control device that switches the exhaust heat recoveryswitching device to the exhaust heat recovery state and operates thecompressor, if a request for heating the air for air conditioning ismade when a temperature of the coolant is lower than a predeterminedtemperature.

In the exhaust heat recovery system according to the aforementionedaspect, if the request for heating is made when the temperature of thecoolant is lower than the predetermined temperature, the control devicemay switch the heater core switching device to the heater core bypassstate.

In the exhaust heat recovery system according to the aforementionedaspect, when the temperature of the coolant is equal to or higher thanthe predetermined temperature, the control device may control theexhaust heat recovery switching device to the discharge state andcontrol the compressor to a stopped state.

In the exhaust heat recovery system according to the aforementionedaspect, when the temperature of the coolant is equal to or higher thanthe predetermined temperature, the control device may control the heatpump switching device to the heat pump bypass state.

As described above, the exhaust heat recovery system according to theaspect of the invention has a great advantage that exhaust heat iseffectively recovered.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and/or further objects, features and advantages of theinvention will become more apparent from the following description ofpreferred embodiment with reference to the accompanying drawings, inwhich numerals are used to represent like elements and wherein:

FIG. 1 is a system block diagram showing an exhaust heat recovery systemaccording to a first embodiment of the invention.

FIG. 2 is a system block diagram showing an exhaust heat recovery systemaccording to a second embodiment of the invention.

FIG. 3 is a side sectional view schematically showing arrangement of aheat-releasing heat exchanger that constitutes a part of the exhaustheat recovery system according to the second embodiment of the inventionin an air conditioner case.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

An exhaust heat recovery system 10 as an exhaust heat recovery systemaccording to a first embodiment of the invention will be described withreference to FIG. 1. FIG. 1 shows a system block diagram (systemflowsheet) of the exhaust heat recovery system 10. As shown in FIG. 1,the exhaust heat recovery system 10 includes a coolant circulationpassage 14 through which an engine coolant is circulated and supplied toan internal combustion engine 12 that produces driving force of avehicle. In the embodiment, a mechanical pump 15 driven by the poweroutput from the internal combustion engine 12 circulates the enginecoolant in the coolant circulation passage 14. Note that, an electricpump may be employed instead of the mechanical pump.

A heater core 16 is provided in the coolant circulation passage 14. Theheater core 16 serves as a heat exchanger that provides hot air forheating the vehicle cabin. In the heater core 16, the engine coolant isused as a heat source. More specifically, the heater core 16 is providedin an air conditioner case (not shown). In the heater core 16, the airintroduced into the air conditioner case is heated by heat transferredfrom the engine coolant so that the heated air is used for heating thevehicle cabin. The vehicle cabin is heated by the heated air introducedinto the vehicle cabin by a blower (not shown), such as a fan.

Further, an exhaust heat recovery heat exchanger 18 is provided in thecoolant circulation passage 14. In the exhaust heat recovery heatexchanger 18, the exhaust heat (energy) of exhaust gas discharged fromthe internal combustion engine 12 is recovered and transferred to theengine coolant. More specifically, in the exhaust heat recovery heatexchanger 18, the exhaust heat of exhaust gas flowing through an exhaustpipe 20, which is connected to an exhaust manifold 12A of the internalcombustion engine 12, is transferred to the engine coolant circulatingin the coolant circulation passage 14. A downstream edge (not shown) ofthe exhaust pipe 20 is opened to atmosphere.

The exhaust heat recovery system 10 further includes a heat pump system22 for recovering the exhaust heat of exhaust gas. In the embodiment,the heat pump system 22 is configured such that the exhaust heat isrecovered and transferred to the engine coolant. The detaileddescription will be given below.

The heat pump system 22 includes a refrigerant circulation passage 24through which a refrigerant is circulated. In the refrigerantcirculation passage 24, a compressor 26, a heat-releasing heat exchanger28, an expansion valve 30, and a heat-absorbing heat exchanger 32 areprovided in this order. In the embodiment, CO2 refrigerant is employedas the refrigerant circulating through the refrigerant circulationpassage 24.

The heat-releasing heat exchanger 28 serves as a refrigerant radiator ofthe heat pump system 22, and as a heat exchanger in which heat istransferred from the CO2 refrigerant to the engine coolant. Morespecifically, in the heat-releasing heat exchanger 28, heat is releasedfrom the CO2 refrigerant circulating through the refrigerant circulationpassage 24 to the engine coolant. The heat-absorbing heat exchanger 32serves as a heat exchanger in which heat is transferred from exhaust gasto the CO2 refrigerant, and is configured so that the CO2 refrigerantabsorbs exhaust heat from exhaust gas (that is, exhaust heat isrecovered and transferred to the CO2 refrigerant). In addition, in theheat pump system 22, the CO2 refrigerant is evaporated in theheat-absorbing heat exchanger 32 so as to recover exhaust heat as latentheat and reduce the temperature and pressure of the CO2 refrigerant.Then the compressor 26 compresses the low-temperature, low-pressure CO2refrigerant to increase the temperature and pressure of the CO2refrigerant, and sends the high-temperature, high-pressure CO2refrigerant to the heat-releasing heat exchanger 28. In theheat-releasing heat exchanger 28, the exhaust heat carried by thehigh-temperature, high-pressure CO2 refrigerant is recovered andtransferred to the engine coolant. The expansion valve 30 expands theCO2 refrigerant to transform the refrigerant into the low-pressureliquid before the refrigerant is supplied to the heat-absorbing heatexchanger 32.

In this way, in the heat pump system 22, exhaust heat of exhaust gas isrecovered and transferred to the engine coolant (i.e., exhaust heat isstored in the engine coolant). In the embodiment, the heat-releasingheat exchanger 28 is disposed, in the coolant circulation passage 14,between the heater core 16 and the exhaust heat recovery heat exchanger18, that is, disposed downstream of the heater core 16 and upstream ofthe exhaust heat recovery heat exchanger 18 in the flowing direction ofthe engine coolant.

The heat-absorbing heat exchanger 32 is disposed downstream of theexhaust heat recovery heat exchanger 18 in the flowing direction ofexhaust gas in the exhaust pipe 20. In addition, a bypass exhaustpassage 20A is provided in the exhaust pipe 20 so as to bypass theexhaust heat recovery heat exchanger 18 and the heat-absorbing heatexchanger 32. An on-off valve 34 is disposed in the bypass exhaustpassage 20A. When the on-off valve 34 opens, exhaust gas mainly flowsthrough the bypass exhaust passage 20A and is discharged to the outside.When the on-off valve 34 closes, exhaust gas flows through the exhaustheat recovery heat exchanger 18 and heat-absorbing heat exchanger 32 inthis order, so that the exhaust gas is recovered in the aforementionedmanner. Note that, the on-off valve 34 functions as an exhaust heatrecovery switching device according to the invention.

The exhaust heat recovery system 10 further includes an ECU (not shown)as a control device of the system. The ECU is electrically connected tothe compressor 26 and the on-off valve 34 so as to operate and stop thecompressor 26, and open and close the on-off valve 34. The ECU receivesoutput signals corresponding to the temperature of the engine coolantfrom a temperature sensor and signals indicative of heating load from anair-conditioning ECU. The ECU controls the compressor 26 and the on-offvalve 34 based on the received signals.

Next, the effects and advantages of the first embodiment will bedescribed.

In the exhaust heat recovery system 10 configured as above, for example,when the temperature of the engine coolant is lower than a predeterminedtemperature (e.g., when the internal combustion engine has juststarted), the ECU closes the on-off valve 34 and operates the compressor26. Then exhaust gas from the internal combustion engine 12 flows intothe exhaust heat recovery heat exchanger 18 and heat-absorbing heatexchanger 32. In the exhaust heat recovery heat exchanger 18, the enginecoolant is directly heated by heat transfer from the high-temperatureexhaust gas to the engine coolant. Further, in the heat pump system 22,the exhaust heat of the exhaust gas recovered in the heat-absorbing heatexchanger 32 is transferred to the CO2 refrigerant, and then the heat issupplied to the engine coolant from the CO2 refrigerant. As a result,the engine coolant is heated.

For example, when the temperature of the engine coolant becomes equal toor higher than the predetermined temperature, the ECU opens the on-offvalve 34 and stops the compressor 26 (to stop the heat pump system 22).In this case, exhaust gas mainly flows through the bypass exhaustpassage 20A and is discharged to the outside.

Since the exhaust heat recovery system 10 is provided with the heat pumpsystem 22, the exhaust heat (thermal energy) that is not recovered inthe exhaust heat recovery heat exchanger 18 can be recovered in the heatpump system 22 and supplied to the engine coolant (that is, to theheater core 16). Further, since the heat pump system 22 is provided withthe heat-absorbing heat exchanger 32 using exhaust gas as a heat source,in other words, the heat source of the heat-absorbing heat exchanger 32is fluid at higher temperature than that of the conventionalconfiguration in which, for example, the outside air or the enginecoolant is used as the heat source, the heat pump efficiency of the heatpump system 22 is high. As a result, exhaust heat of exhaust gas that isnot recovered in the exhaust heat recovery heat exchanger 18 can beefficiently recovered in the heat pump system 22.

The heat-absorbing heat exchanger 32 is disposed downstream of theexhaust heat recovery heat exchanger 18 in the flowing direction ofexhaust gas. Therefore, the CO2 refrigerant can be prevented from beingoverheated, i.e. overexpanded due to the high-temperature exhaust gas.In other words, the heat pump system 22 having high heat recoveryefficiency is protected against the high temperature. Further, since theheat-absorbing heat exchanger 32 uses exhaust gas as the heat source,there will be no problem of frost formation, which is a concern when theoutside air is employed as the heat source, for example.

In the exhaust heat recovery system 10, since the exhaust heat recoveredin the heat pump system 22 is used to increase the engine coolanttemperature, the internal combustion engine 12 is quickly warmed up, andheating performance using the heater core 16 can be improved. Thus, evenimmediately after the start of the internal combustion engine 12, goodheating performance can be achieved.

Moreover, in the heat pump system 22, the heat-releasing heat exchanger28 is disposed so as to release the exhaust heat into the engine coolantat the lowest temperature, that is, the coolant before the exhaust heatis recovered in the exhaust heat recovery heat exchanger 18 provided inthe coolant circulation passage 14 and after the exhaust heat isreleased from the coolant in the heater core 16. Therefore, the heatpump efficiency is high, and the exhaust heat can be more efficientlyrecovered. Thus, the entire exhaust heat recovery system 10 has improvedheat recovery efficiency.

Based on what is described above, in the vehicle to which the exhaustheat recovery system 10 is applied, the heating performance is improvedwhile maintaining good fuel efficiency. That is, the heating performancecan be improved without deteriorating fuel efficiency. Morespecifically, in the conventional heat recovery system configuredwithout the exhaust heat recovery heat exchanger 18 and the heat pumpsystem 22, it is necessary to increase the idling engine speed toquickly heat the coolant in the coolant circulation passage 14 or tochange the shift timing of the transmission in order to improve theheating performance (increase the temperature of the engine coolant).However, in the exhaust heat recovery system 10, since the exhaust heatis efficiently recovered and supplied to the engine coolant, it is nolonger necessary to increase the idling engine speed nor change theshift timing, thus contributing to improvement of the heatingperformance without deteriorating fuel efficiency. At the same time, theengine is further quickly warmed up while maintaining good fuelefficiency.

Thus, in the exhaust gas heat recovery system 10 according to the firstembodiment, the exhaust heat can be efficiently recovered.

Next, an exhaust heat recovery system 50 according to a secondembodiment of the invention will be described with reference to FIGS. 2and 3. The same components and portions in the second embodiment asthose in the first embodiment will be denoted by the same referencenumerals, and therefore the detailed description thereof will not berepeated.

FIG. 2 shows a system block diagram (system flowsheet) of the exhaustheat recovery system 50 according to the second embodiment of theinvention. As shown in FIG. 2, the exhaust heat recovery system 50includes a heat pump system 52, instead of the heat pump system 22. Thisdistinguishes the exhaust heat recovery system 50 from the exhaust heatrecovery system 10 according to the first embodiment.

The heat pump system 52 is basically the same as the heat pump system22, except that the heat pump system 52 includes a heat-releasing heatexchanger 54 in which heat is transferred from the CO2 refrigerant tothe air used for air conditioning, instead of the heat-releasing heatexchanger 28 in which heat is transferred from the CO2 refrigerant tothe engine coolant. Hence, in the heat pump system 52, the exhaust heatrecovered in the heat-absorbing heat exchanger 32 is released into theair in the heat-releasing heat exchanger 54 so as to produce hot air forheating the vehicle cabin. In the embodiment, air heating does notrequire heat transfer through the engine coolant nor the heater core 16.

As shown in FIG. 3, the heat-releasing heat exchanger 54 and the heatercore 16 are disposed in the air conditioner case 56. The heat-releasingheat exchanger 54 and the heater core 16 form a vehicle air conditioner58. An example of the vehicle air conditioner 58 will be supplementarilydescribed below.

As shown in FIG. 3, the air conditioner case 56 opens at its both ends.An outside air door 62 and recirculating air doors 64 are formed on oneend of the air conditioner case 56. The outside air door 62 and therecirculating air doors 64 are open and closed byrecirculation/ventilation switching dampers 60. On the other end of theair conditioner case 56, a plurality of air outlets 68 are provided toopen toward the vehicle cabin, and opened and closed by mode selectordampers 66 when appropriate. The air outlets 68 include, for example, adefroster outlet 68A, a side/center resister air outlet 68B, and a floorair outlet 68C. Thus, the heated air is allowed to blow into the vehiclecabin through at least one of the air outlets 68 at the desiredposition(s) using the mode selector dampers 66.

Further, a blower 70 is provided downstream of the outside air door 62and the recirculating air doors 64 in the air conditioner case 56. Theblower 70 sucks in the air into the air conditioner case 56 through theoutside air door 62 or the recirculating air doors 64 and sends thesucked air toward the air outlets 68. An evaporator 72 for cooling theair for air conditioning is provided downstream of the blower 70 in theair conditioner case 56. The evaporator 72 may constitute arefrigeration cycle (not shown). When the refrigeration cycle isperformed, the refrigerant is evaporated in the evaporator 72 by heattransfer from the air to the refrigerant so as to get rid of latent heatfrom the air.

An air mix damper 74 and the heater core 16 are provided downstream ofthe evaporator 72 in the air conditioner case 56. The air downstream ofthe evaporator 72 is guided into the heater core 16 based on, forexample, the opening degree of the air mix damper 74, and then the airis heated. Then, after being mixed with the unheated air that has notpassed through the heater core 16, the heated air is sent toward the airoutlets 68. The air mix damper 74 can adjust the ratio of the amount ofair that flows through the heater core 16 to the amount of air that doesnot flow through the heater core 16 between the ratio in the state wheresubstantially all the air from the evaporator 72 flows through theheater core 16, and the ratio in the state where substantially all theair from the evaporator 72 does not flow through the heater core 16 (thelatter state will be hereinafter referred to as “heater core bypassstate”). Note that, the air mix damper 74 functions as a heater coreswitching device according to the invention.

An air mix damper 76 and a heat-releasing heat exchanger 54 are provideddownstream of the heater core 16 in the air conditioner case 56. The airdownstream of the evaporator 72 (the heater core 16) is guided into theheat-releasing heat exchanger 54 based on, for example, the openingdegree of the air mix damper 76, and then, the air is heated. Then,after being mixed with the unheated air that has not passed through theheat-releasing heat exchanger 54, the heated air is sent toward the airoutlets 68. The air mix damper 76 can adjust the ratio of the amount ofair that flows through the heat-releasing heat exchanger 54 to theamount of air that does not flow through the heat-releasing heatexchanger 54 between the ratio in the state where substantially all theair from the evaporator 72 flows through the heat-releasing heatexchanger 54 and the ratio in the state where substantially all the airfrom the evaporator 72 does not flow through the heat-releasing heatexchanger 54 (the latter state will be hereinafter referred to as “heatpump bypass state”). Note that, the air mix damper 76 functions as aheat pump switching device according to the invention.

The aforementioned exhaust heat recovery system 50 and the vehicle airconditioner 58 are integrally (synchronously) controlled by a controldevice (not shown). When the heat pump system 52 is operated, the airmix damper 74 is placed in the heater core bypass state where the air isnot heated in the heater core. When the heat pump system 52 stops, theair mix damper 76 is placed in the heat pump bypass state.

The configurations of other parts of the exhaust heat recovery system 50are the same as those of the exhaust heat recovery system 10. Next, theeffects and advantages of the exhaust heat recovery system 50 accordingto the second embodiment will be described below.

In the exhaust heat recovery system 50 configured as above, for example,when the temperature of the engine coolant is lower than a predeterminedtemperature (e.g., when the internal combustion engine 12 has juststarted), the ECU closes the on-off valve 34. With the on-off valve 34closed, the exhaust heat of exhaust gas is recovered and transferred tothe engine coolant in the exhaust heat recovery heat exchanger 18.Accordingly, the internal combustion engine 12 is quickly warmed up. Inparticular, if the air mix damper 74 is switched to the heater corebypass state, heat release from the heater core 16 is suppressed, andthe internal combustion engine 12 is further quickly warmed up. When thetemperature of the engine coolant becomes equal to or higher than thepredetermined temperature, for example, the ECU opens the on-off valve34. In this case, the exhaust gas mainly flows through the bypassexhaust passage 20A and is discharged to the outside.

Further, in the exhaust heat recovery system 50, for example, if arequest for heating the vehicle cabin is made when the engine coolanttemperature is lower than the predetermined temperature (e.g., when theinternal combustion engine 12 has just started), the ECU closes theon-off valve 34 and operates the compressor 26. Then, exhaust gas fromthe internal combustion engine 12 flows into the exhaust heat recoveryheat exchanger 18 and the heat-absorbing heat exchanger 32. In theexhaust heat recovery heat exchanger 18, the engine coolant is directlyheated by heat transfer from the high-temperature exhaust gas. Thus, theinternal combustion engine 12 is further quickly warmed up.

In the heat pump system 52, the exhaust heat of exhaust gas recovered inthe heat-absorbing heat exchanger 32 is transferred to the CO2refrigerant, and is then supplied from the CO2 refrigerant to the airintroduced into the air conditioner case 56. Thus, the air used forheating the vehicle cabin is heated. In this case, the air mix damper 74is placed in the heater core bypass state, and the air is not heated inthe heater core 16. The air is further heated by heat transferred fromthe CO2 refrigerant in the heat-releasing heat exchanger 54, and usedfor heating the vehicle cabin.

For example, when the temperature of the engine coolant becomes equal toor higher than the predetermined temperature, the ECU opens the on-offvalve 34 and stops the compressor 26 (to stop the heat pump system 52).In this case, exhaust gas mainly flows through the bypass exhaustpassage 20A and is discharged to the outside. The air mix damper 74 isswitched to the state where the air is introduced to the heater core 16,and the air mix damper 76 is switched to the heat pump bypass state.Therefore, after the engine coolant temperature increases, the heatercore 16 (the engine coolant) is used as the heat source for heating thevehicle cabin.

Since the exhaust heat recovery system 50 is provided with the heat pumpsystem 52, the exhaust heat unrecovered in the exhaust heat recoveryheat exchanger 18 can be recovered in the heat pump system 52. Further,since the heat pump system 52 includes the heat-absorbing heat exchanger32 having exhaust gas as a heat source, in other words, the heat sourceof the heat-absorbing heat exchanger 32 is fluid at higher temperaturethan that of the conventional construction in which, for example, theoutside air and engine coolant are used as heat sources, the heat pumpefficiency is high. As a result, exhaust heat of exhaust gas unrecoveredin the exhaust heat recovery heat exchanger 18 can be efficientlyrecovered in the heat recovery system 50.

In the exhaust heat recovery system 50, the exhaust heat recovered inthe heat-absorbing heat exchanger 32 is supplied to the air used for airconditioning (the heat-releasing heat exchanger 54). In theheat-releasing heat exchanger 54, the air is directly heated withoutheat transfer through the engine coolant (the heater core 16).Therefore, the heated air, that is, hot air for air conditioning can beprovided before the temperature of the engine coolant is increased. Thatis, at the cold start, etc., hot air can be provided within a shortperiod of time from the engine start, thus contributing to improvementof quick heating performance.

When the heat pump system 52 is operated in the manner described above,the air mix damper 74 suppresses heat release from the heater core 16into the air. Therefore, the temperature of the engine coolant in theheater core 16 is prevented from decreasing, and is increased within ashort period of time. That is, the internal combustion engine 12 can bequickly warmed up without deteriorating (and while improving) theheating performance of the vehicle air conditioner.

Since the heat-absorbing heat exchanger 32 is disposed downstream of theexhaust heat recovery heat exchanger 18 in the flowing direction ofexhaust gas, the CO2 refrigerant is prevented from being overheated,i.e. overexpanded due to the high-temperature exhaust gas. In otherwords, the heat pump system 52 having high heat recovery efficiency isprotected against the high temperature. Further, since exhaust gas isused as the heat source in the heat-absorbing heat exchanger 32, therewill be no problem of frost formation, which is a concern when theoutside air is employed as the heat source, etc.

Based on what is described above, in the vehicle to which the exhaustheat recovery system 50 is applied, the heating performance is improvedwhile maintaining good fuel efficiency, as in the case of the exhaustheat recovery system 10 according to the first embodiment. Thus, theexhaust heat can be efficiently recovered in the exhaust heat recoverysystem 50 according to the second embodiment.

1. An exhaust heat recovery system comprising: an exhaust heat recoveryheat exchanger in which heat is transferred from exhaust gas dischargedfrom an internal combustion engine to a coolant for cooling the internalcombustion engine; and a heat pump including: a circulation passage fora refrigerant; a heat-absorbing heat exchanger in which the heat istransferred from the exhaust gas to the refrigerant; and aheat-releasing heat exchanger in which the heat is transferred from therefrigerant to the coolant, directly to a circulation passage for thecoolant, wherein said heat-releasing heat exchanger is disposed upstreamof exhaust heat recovery heat exchanger in the circulation passage forthe coolant.
 2. The exhaust heat recovery system according to claim 1,wherein the heat-absorbing heat exchanger of the heat pump is disposeddownstream of the exhaust heat recovery heat exchanger in a flowingdirection of the exhaust gas.
 3. The exhaust heat recovery systemaccording to claim 1, wherein the coolant is circulated by a pump drivenby a power from the internal combustion engine.
 4. The exhaust heatrecovery system according to claim 1, further comprising: an exhaustheat recovery switching device switchable between an exhaust heatrecovery state where exhaust heat of the exhaust gas is recovered and adischarge state where the exhaust gas is discharged to an outside,wherein the exhaust gas flows through the exhaust heat recovery heatexchanger and the heat-absorbing heat exchanger by switching the exhaustheat recovery switching device from the discharge state to the exhaustheat recovery state.
 5. The exhaust heat recovery system according toclaim 4, further comprising: a control device that switches the exhaustheat recovery switching device to the exhaust heat recovery state when atemperature of the coolant is lower than a predetermined temperature,and switches the exhaust heat recovery switching device to the dischargestate when the temperature of the coolant is equal to or higher than thepredetermined temperature.
 6. The exhaust heat recovery system accordingto claim 4, wherein the heat pump includes a compressor that compressesthe refrigerant to which the heat has been transferred in theheat-absorbing heat exchanger, said exhaust heat recovery system furthercomprising: a control device that switches the exhaust heat recoveryswitching device to the exhaust heat recovery state and operates thecompressor when the temperature of the coolant is lower than thepredetermined temperature.
 7. The exhaust heat recovery system accordingto claim 6, wherein the control device controls the exhaust heatrecovery switching device to the discharge state and controls thecompressor to a stopped state when the temperature of the coolant isequal to or higher than the predetermined temperature.
 8. The exhaustheat recovery system according to claim 1, further comprising: a heatercore that is disposed upstream of the exhaust heat recovery heatexchanger in the circulation passage for the coolant and heats air forair conditioning, wherein the heat-releasing heat exchanger of the heatpump is disposed downstream of the heater core in the circulationpassage for the coolant.
 9. An exhaust heat recovery system comprising:an exhaust heat recovery heat exchanger in which heat is transferredfrom exhaust gas discharged from an internal combustion engine to acoolant for cooling the internal combustion engine; and a heat pumpincluding: a circulation passage for a refrigerant; a heat-absorbingheat exchanger in which the heat is transferred from the exhaust gas tothe refrigerant; and a heat-releasing heat exchanger in which the heatis transferred from the refrigerant to an air for air conditioning,wherein said exhaust heat recovery system further comprises: a heat pumpswitching device switchable between an air supply state where the airfor air conditioning is supplied to the heat-releasing heat exchangerand a heat pump bypass state where the air for air conditioning is notsupplied to the heat-releasing heat exchanger.
 10. The exhaust heatrecovery system according to claim 9, further comprising: a heater corethat is provided in the circulation passage for the coolant and heatsthe air for air conditioning; and a heater core switching deviceswitchable between an air supply state where the air for airconditioning is supplied to the heater core and a heater core bypassstate where the air for air conditioning is not supplied to the heatercore.
 11. The exhaust heat recovery system according to claim 9, whereinthe heater core switching device is switched to the heater core bypassstate when the heat pump is operated, and the heat pump switching deviceis switched to the heat pump bypass state when the heat pump is stopped.12. The exhaust heat recovery system according to claim 9, furthercomprising: an exhaust heat recovery switching device switchable betweenan exhaust heat recovery state where the exhaust heat of the exhaust gasis recovered and a discharge state where the exhaust gas is dischargedto an outside; and a control device that switches the exhaust heatrecovery switching device to the exhaust heat recovery state andswitches the heater core switching device to the heater core bypassstate when a temperature of the coolant is lower than a predeterminedtemperature.
 13. The exhaust heat recovery system according to claim 9,wherein the heat pump includes a compressor that compresses therefrigerant to which the heat has been transferred in the heat-absorbingheat exchanger, said exhaust heat recovery system further comprising: anexhaust heat recovery switching device switchable between an exhaustheat recovery state where exhaust heat of the exhaust gas is recoveredand a discharge state where the exhaust gas is discharged to an outside;and a control device that switches the exhaust heat recovery switchingdevice to the exhaust heat recovery state and operates the compressor ifa request for heating the air for air conditioning is made when atemperature of the coolant is lower than a predetermined temperature.14. The exhaust heat recovery system according to claim 13, wherein thecontrol device switches the heater core switching device to the heatercore bypass state if the request for heating is made when thetemperature of the coolant is lower than the predetermined temperature.15. The exhaust heat recovery system according to claim 13, wherein thecontrol device controls the exhaust heat recovery switching device tothe discharge state and controls the compressor to a stopped state whenthe temperature of the coolant is equal to or higher than thepredetermined temperature.
 16. The exhaust heat recovery systemaccording to claim 15, wherein the control device controls the heat pumpswitching device to the heat pump bypass state when the temperature ofthe coolant is equal to or higher than the predetermined temperature.